This mechanical clock was built in 1804. At this period, most
clocks kept time through the regular swinging of a pendulum.
However, instead of a pendulum, this clock uses a rolling ball to
provide the timekeeping mechanism.
How does a rolling ball clock work?
You can see a video of the clock
in motion here. Note how the brass ball zigzags down a track,
which is laid out on a plate. When it reaches the end of the track,
the ball hits a lever and a spring raises the end of the plate.
This sends the ball rolling back the other way and also advances
the hands of the clock by 15 seconds, 5,760 times a day.
Does it keep good time?
This idea sounds good in theory, but unfortunately, it doesn't
work all that well in practice. Ideally, the ball would always take
exactly the same amount of time to roll down the track, but it
doesn't. Any trace of dust will get in the way of the ball rolling
and slow it down – and even under a glass dome or in a case there
is dust. In addition, the metal expands or contracts with changes
in temperature, altering the length of the track and size of the
ball.
Who invented the rolling ball clock?
Sir William Congreve (1772-1828) claimed the credit for
inventing this rolling ball clock and took out a patent for the
design in 1808. Interestingly, however, our clock was made by
the Edinburgh clock maker Robert Bryson four years earlier, in
1804.
Most famous for designing the Congreve rocket, which was used
during the Napoleonic wars and the American War of Independence,
Congreve was a prolific inventor.
Can a rolling ball clock be made to keep good time?
‘A most complicated and troublesome machine…’
This was how staff at Buckingham Palace described a rolling ball
clock they were tasked with keeping to time in 1837.
Assistant conservator of technology Darren Cox has been
wrestling with our Congreve clock to try to persuade it to keep
time reliably. He has reset it to the correct time every working
day for two months.
The chart below shows how much time it lost or gained in that
period – up to 45 minutes a day! (The vertical lines indicate
adjustments to the clock's time keeping.)
Darren explains:
“The Bryson rolling ball clock is designed to last two weeks
after one winding. The spring on this clock is a little weak with
age, not enough to affect any other clock but a rolling ball clock
needs so much energy to keep it going that on occasions it won’t
last the two weeks.
“The clock and the tilting table (front and
back) need to be totally level. I have found that, even with the
temporary cover I have made for it to keep the dust out, the metal
ball needs to be polished at least once every two weeks to remove
debris.
“However, the biggest task in getting this clock to work was
adjusting all the levers and the table that had been bent and
twisted previously. It takes very little to stop the ball from
rolling and if it doesn’t roll fast enough it will not be able to
unlock the train of wheels and lift the table.
“The only other rolling ball clock I know well kept time to
about 15 minutes a day in a very stable environment. Our
engineering workshop is far from stable and this clock is very
susceptible to temperature and humidity. This and the fact that I
have not been on hand regularly to attend to the clock’s every need
is reflected in the time keeping I recorded. I noted yesterday that
the clock was five minutes slow; this morning it was two hours
slow!
“I think if the conditions are good enough we should be able to
get accuracy for this clock, give or take about 15 minutes a day,
but unless the case it’s kept in is perfect at keeping the dust
out, the time keeping will be a lot wilder.”
The slideshow below shows the clock in the Conservation
workshop.
Where is the clock displayed?
The Congreve clock is displayed in the
Earth in Space gallery in the National
Museum of Scotland. But don’t rely on it to tell you the exact time
– we will only be resetting it once a week, so it may lose or gain
by hours!